ΔE 15.1, loose: different enough that you should pick on purpose. Warpfiend Grey is much more muted than Matt Purple. They are different kinds of paint: AMMO by Mig Matt Purple is an ink, glaze or transparent paint and Citadel Warpfiend Grey is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Warpfiend Grey is much more muted than Matt Purple. They are different kinds of paint.
The same difference applies the other way round: Matt Purple is much more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Matt Purple on the colour wheel See Warpfiend Grey on the colour wheel
All matches: Matt Purple equivalents · Warpfiend Grey equivalents